Bias for electron beam equipment



Jan. 28, 1958 w; IWESTENDORP BIAS FOR ELECTRON BE'IAM EQUIPMENT FiledJune 27, 1955 Inventor.- WI'IIem 7 Wesendorp,

4 j His Attorney.

atet

2,821,655 Patented Jan. 28, 1958 teal Willem F. Westendorp, Schenectady,N. Y., assignor to General Electric Company, a corporation of New YorkApplication June 27, 1955, Serial No. 518,199

11 Claims. (Cl. 315-14) This invention relates to apparatus for biasingelectron beam equipment.

While this invention may take a variety of forms, it is ideally suitedfor biasing high voltage electron beam equipment such as cathode rayapparatus, providing electrons having energies of the order of 1 millionvolts or more, and X-ray apparatus and, by way of example, isparticularly described in connection with these forms of equipment.

In conventional equipment for providing high energy electrons in theform of an electron beam, an electron gun, including an electronemitting filament and a control or focusing electrode, provideselectrons. Electrons from the filament pass through the controlelectrode and are then accelerated by high potentials applied to aplurality of accelerating electrodes to provide a beam of high energyelectrons.

One variety of electron beam equipment utilizes a resonance transformerwherein successively higher accelerating potentials, from successivetaps on a resonance transformer, are applied to accelerating electrodesarranged along the beam path. In this form of apparatus, electrons areaccelerated by the positive portions of the voltage wave from theresonance transformer. It is apparent that electrons accelerated withthis form of equip ment have a wide range of energies which range fromthose energies associated with the relatively low positive potentialportions of the accelerating voltage wave to those associated with andaccelerated by the peak portion of the accelerating voltage wave. Sincethe accelerating wave has a substantially sine wave form it. is furtherapparent that the electron energy distribution follows the samefunction.

Many of the resulting low energy electrons cannot penetrate the endwindow of electron beam apparatus. These electrons do no useful work andresult in undesirable heating of the end window. In electron beamequipment, it is general practice to focus the electrons in the beamelectrostatically and/or electromagnetically. The wide range of electronvelocities resulting from the range of accelerating voltages rendersfocusing of the beam extremely difficult. Since the electron energiesvary over a wide range and many of the electrons cannot be utilized, theefficiency of the electron beam tube equipment is relatively low.

Therefore, it is apparent that it would be particularly desirable ltOprovide apparatus for biasing electron beam equipment so that a beam ofelectrons having substantially the same energy is obtained. With such abeam substantially all of the electrons may be accelerated so that theyare able to penetrate the end window of the tube and so that only thoseelectrons are present which have nearly the maximum range of penetrationfor useful work. The efliciency of such equipment is improved and theheating of the end window is substantially reduced. In addition, sincethe electrons have nearly the same energy, the electron current densityis more uniformly distributed over the cross-section of the beam and thebeam diameter can be more easily adjusted by means of a focusing coil.

Therefore, an object of this invention is to provide improved apparatusfor biasing electron beam equipment so that a beam of electrons havingsubstantially the same energy is obtained.

It is also an object of this invention to provide improved apparatus forbiasing electron beam equipment so that beam current flows in saidequipment during a short portion only of the cycle of the acceleratingvoltage applied to the equipment and so that electrons havingsubstantially the same energy are obtained.

According to an aspect of this invention there is provided a novelsolution to the problem of obtaining, in electron beam equipment,electrons having substantially the same energy. This solution is in theform of apparatus for biasing electron beam equipment, of the typeincluding an electron gun having a control electrode and a source ofvoltage applied to the equipment for accelerating electrons from thegun, comprising a source of peaked voltage applied to the controlelectrode. The peaked voltage has the proper phase, amplitude, andperiodicity with respect to the accelerating voltage so that electronsflow in the equipment during a short portion only of the cycle of theapplied accelerating voltage, thereby providing a beam of electronshaving substantially the same energy.

Other objects and important aspects of this invention will becomeapparent from the specification and claims when taken with the figuresof the drawing, wherein Figures 1 and 1a illustrate an embodiment ofthis invention and wave forms associated therewith; Figures 2 and 2aillustrate another embodiment of this invention and associated waveforms; Figure 3 illustrates a preferred embodirnent of this invention;and Figure 4 illustrates wave forms useful in explaining the operationof the embodiment illustrated in Figure 3.

Figure 1 illustrates a peaking transformer including core members 10 and11 of magnetic material. Core 10 is provided with a gap 12 and core 11includes consider ably less magnetic material than core 10. Therefore,core 11 saturates at a much lower value of magnetizing current than core10. A winding 13 provides the magnetizing force to cause flux tocirculate in cores 1d and 11. Coil 13 is energized by energy source 14which may consist of end turns on a high voltage transformer. Since thereluctance of the magnetic circuit formed by core member 10 is reflectedback to energizing coil 13 and to energy source 14, the current in coil13 is conveniently adjusted by varying the spacingof air gap 12. Thephase of the energizing current with respect to the current applied tothe transformer energizing coil 14 is adjusted by variable resistor 15.i

A secondary winding 16 is coupled to core 11 only. One end of coil 16 isconnected to filament 17 through lead 18 and the other end of coil 16 isconnected through self-biasing network 19 to control or focusingelectrode 20. The self-biasing network consists of resistor 21 andcapacitor 22. The equipment is provided with transformer secondary 23which provides high potential accelerating voltages to acceleratingelectrodes 24 and 25 which are arranged along the beam path and ininsulating housing 26 of the beam equipment.

The filament is continuously heated and electrons from the filament areaccelerated by potentials applied to electrodes 24 and 25 fromtransformer secondary 23. Some electron current is collected byelectrode 20 whenever it becomes positive with respect to the filament17. After approximately one cycle of accelerating voltage, the positivepeak voltage output of coil 16 charges capacitor 22 so that the platenearest the peaking transformer is positive and the plate furthest fromthe transformer is negative thereby applying a negative bias to controlelectrode 20. Resistor 21 has a high resistance so that a relativelylong time, compared to the period of the applied accelerating potential,is required for charge to leak off capacitor 22. Therefore, asubstantially constant negative bias is applied to electrode 29, inaddition to the alternating voltage from winding 16 Since transformersecondary core member 11 is saturated during a major portion of themagnetizing force applied by coil 13 the voltage output of coil 16 has apeaked characteristic centered about the region of most rapid change offiux which is substantially 180 out, of phase with the energizingcurrent applied to coil 13. The phase of the peaked voltage output ofcoil 16 is controlled by adjusting variable resistor 15 so that thepositive peaks occur during the positive half cycle of the substantiallysine wave accelerating voltage applied by coil 23 and during the periodwhen the applied accelerating potential is at a maximum value. Thenegative bias, provided by network 19, and the peaked voltage from coil16 bias electrode 20 so that electrons flow in the beam equipment duringa short portion of the positive half cycle of the accelerating voltageonly and so that substantially no electrons fiow during the remainder ofthe accelerating voltage cycle.

Figure 1a illustrates the voltage characteristics associated with theapparatus illustrated in Figure 1. Line 18' is representative of thebias potential applied to electrode 29. Negative peaks 16 and positivepeaks 16 are representative of the potential observed across transformersecondary 16 when superimposed upon the negative bias providedby network19. Curve 23 is representative of the-accelerating potential applied toone or more of the accelerating electrodes 24- and 25. The curve 27 isrepresentative of electron current that is permitted to flow,

It will be noted that electrode 2.0, which focuses the electron beam andacts as a control electrode, is located in proximity to the filament sothat it has a much greater control effect upon the electrons emitted byfilament 17 than do the accelerating electrodes; for example, 100 timesthe control exerted by electrodes 24 and 25. In view of this relativespacing of the accelerating electrodes and electrode 20, the criticalbias voltage line 28 may be drawn to approximately the same scale as thebiasvoltage applied to control electrode 20. If the bias potential onelectrode 20 exceeds the critical bias, represented by line 28, electroncurrent flows. that accelerating voltage curve 23' is drawn toapproximately A of its true value inamplitude in order to provide asimplified illustration. Where bias voltage line 18' crosses criticalbias voltage line 28, electrons from filament 17 pass electrode 20 andare accelerated. Thus; it is apparent that the apparatus illustrated inFigure l limits the beam current to a short portion only of the cycle ofthe applied accelerating voltage. It will be noted that negative peaks16, serve no useful purpose since theyoccur during the negative halfcycle of the accelerating potentiah These negative peaks necessitateheavy insulation between filament 17 and control electrode 20.

Self-biasing networks have, prior to this invention, been utilized in aneffort to. limit the beam current to short periods of the acceleratingpotential cycle; however, these networks provide a substantiallysinusoidal bias to the control electrode-so that theresulting currentflow isnot sharply cut off. This results ina beam; cur rentcharacteristic having sloped sides rather than the square wavecharacteristicv having steep substantially ver-. ticalsides 27, asillustrated in Figure la.

Inorder to, overcomethe necessityifor heavyinsulanon between thefilament 17, and electrode. 20,. a rectifier:

It is also noted,

4 may be introduced into the bias circuit to eliminate the negativeportion of the output of the peaking transformer secondary coil 16.Figure 2 illustrates such an apparatus wherein components which aresimilar to those illustrated in Figure 1 are designated by the samereference numerals. The apparatus includes a rectifier 29 and resistor30 connected to eliminate the negative peaks from the output of peakingtransformer secondary winding 16.

Figure 2a illustrates the current characteristic obtainable with thisapparatus. It is apparent from these curves that the maximum potentialdifference between electrode 20 and filament 17 is greatly reduced sothat considerably less insulation between these two members isnecessary. With the exception of the operation of the rectifierresistorcircuit 29-3tl to eliminate the negative peaks from the output of coil16, the operation of the apparatus illustrated in Figure 2 is the sameas the operation of the apparatus illustrated in Figure 1.

It is noted that in the apparatus illustrated in Figures l and 2, afixed negative bias, from a direct current source for example, may besubstituted for the self-biasing network 19. In addition, a separatesource of variable biasing potential can be applied to the peakingtransformer rather than utilizing end turns coupled to the source ofaccelerating potential. For example, any source of peaked voltage,having the same periodicity as the accelerating voltage, and combinedwith an appropriate negative bias source, may be applied to electrode24), to obtain an electron beam with electrons having substantially thesame energy.

Figure 3 illustrates apparatus which provides the desired sharp cut ofielectron beam tube bias without the necessity of utilizing a separatebiasing network or rectifying circuit. There is illustrated a peakingtransformer with normally saturated core 31 and normally unsaturatedcore 32. Core 32 has a high reluctance gap 33. The transformer isprovided with an energizing winding 34, a peaked output winding 35,linking core 31 only, and a tertiary winding 36 linking cores 31 and 32.Windings 35 and 36 are connected in series phase opposition and areconnected across filament 17 and focusing electrode 2th through variableratio transformer 37, transformer 38, and current limiting resistor 39.

Energization forthe peaking transformer is provided by end turns 49 ofaccelerating transformer 41 through variable ratio transformer 52 andvariable resistor 43. Filament heater potential is provided by end turns44 of transformer 41 and accelerating potential is applied toacceleratingelectrode 45 from tap 46.

Gap 33, variable. resistor 43 and variable ratio transformer 42 controlthe amplitude and phase of the energizing current applied to coil 44,and thereby the amplitude, width and phase of the peaked voltage acrossWinding 35. The wave form of voltage across tertiary coil 36 issubstantially the same as that applied to coil 34. Variable ratiotransformer 37 controls the amplitude of the potentialapplied totransformer 38 and thereby the magnitude of the bias applied betweenfilament 17 and electrode 26. it is readily apparent that variable ratiotransformer 42, variable resistor'43, variable ratio transformer 37 andtransformer 38 can be eliminated for any specific application throughproper'design of the transformer elements or replacement of the variablecomponents with fixed components.

A consideration of the-curves shown in Figure 4 will aid inunderstanding the operation of this embodiment. Curve 47 isrepresentative of the voltage appearing across the terminals of coil 36.Curve 48 is representative of the flux in core 31. It'will be noted thatthis flux is out of phase with the/voltage induced in coil 36 and thata-voltage output isobserved across coil 35 only during theperiodsof;rapid.flux change. Thisresults in peaked voitagecharacteristic curve49 having negative peaks 49. and positive peaks 49".

Voltage, curve 47 has ,been .shiftedlSO? in phasefrom its actual phaserelationship with respect to flux characteristic 48 so that the seriesconnected phase opposition resultant voltage obtainable from thecombined outputs of windings 35 and 36 can be illustratedeasily byadding these voltages to obtain curve 50. Curve 50 has positive voltagepeaks 50 which are adjusted to correspond with the positive peakportions of the high potential accelerating voltage illustrated by curve51. Portions 51 are the only portions during the accelerating potentialcycle when the electrons flow from filament 17.

A consideration of the curves illustrated in Figure 4 indicates thatrelatively low negative peaks are present so that only light insulationis necessary between filament 17 and electrode 20. The maximum voltagebetween filament 17 and electrode 20 is only slightly larger than thatneeded for proper biasing.

The turns ratio of coil 35 to the turns ratio of coil 36 is selected sothat the peak induced voltage of coil 35 exceeds that of coil 36 byapproximately 40%. Thus, when these two windings are connected in phaseopposition, the voltage produced as a bias permits electrons fromfilament 17 to flow in the apparatus in the middle only of the usefulhalf cycle of the accelerating potential from transformer 41. During theinverse half cycle of the accelerating potential, the focusing electrodebecomes positive with respect to filament 17. Resistor 39 preventsexcessive current flow, during this period and during the useful halfcycle of the accelerating voltage, which would spoil the bias wave form.

In the practice of this invention, limiting the electron current flow tothe highest 30 to 80 of the positive half cycle of the acceleratingpotential produces beneficial results. That is, from approximately plusor minus 15 on either side of the peak value of the sinusoidalaccelerating potential to approximately plus or minus 40 on either sideof this peak value. For example, with beam current flow limited to plusor minus 40 on either side of the peak value, the lowest speed electronsare accelerated by approximately 76% of the potential applied to theelectrons accelerated by the peak value of the accelerating potential,i. e. the cosine of 40' is approximately 0.76. If the flow of beamcurrent is limited to plus or minus 15 either side of the peak value,the lowest speed electrons are accelerated by approximately 97% of thepotential applied to electrons accelerated by the peak value of theaccelerating potential.

As the period during which electrons are accelerated is reduced, thefilament temperature must be increased in order to provide a readysupply of electrons to be rapidly drawn away from the filament duringthis short period. In view of the cosine relationship of the emissionperiod it is apparent that little is gained by decreasing the emissionangle to less than 30, particularly in view of the necessity ofcontinuously maintaining the filament at a high operating temperaturethereby reducing its life.

Apparatus of the type illustrated in Figure 3 has been utilized withequipment for accelerating electrons to energies of the order of l to1.5 million volts. This specific apparatus includes a main magnetic corehaving a cross-section of inch by 1 /8 inches with 2 air gaps of ,5 inchformed of transformer steel laminations. The saturated core is made ofstrip steel laminations with a cross-section of inch by 1 /2 inches. Themain energizing winding 34 is wound with 350 turns, coil 35 has 974turns, and coil 36 has 700 turns. The step up transformer 38 is rated at100 volts to 2000 volts at 180 cycles per second.

By adjusting variable ratio transformer 42, the saturation of core 31 iscontrolled and thereby the width of the induced voltage peaks obtainablefrom winding 35. By adjusting these voltage peaks the electron currentinterval in the electron beam equipment is controlled. The amplitude ofthe bias voltage is controlled by variable ratio transformer 37 and afinal bias voltage of the order of 1.000 volts is obtained from smallstep up transformer 38.

Resistor 39 for limiting the current during the inverse half cycle ofthe accelerating voltage is l megohm and phase control variable resistor43 is 25 ohms.

In view of the foregoing, it is apparent that the apparatus of thisinvention pemits beam current flow in electron beam equipment to occuronly during a short interval of the cycle when the sinusoidalaccelerating voltage is highest. This type of operation results in abeam of high energy electrons having substantially the same energy,thereby eliminating those electrons which cannot penetrate the endwindow. Electrons flow only during those periods when voltage is presentto accelerate them to have nearly the maximum range of penetration foruseful work. Thus, the efficiency of the beam apparatus is improved,beam focusing is made easier, the utility of the beam is increased, andthe heating of the window is reduced.

When apparatus in accordance with this invention is applied to highvoltage X-ray equipment, the efficiency is raised, the X-ray output isincreased and the quality of the radiation is improved, since all of theelectrons have approximately the same energy. At 1 million volts peak, a51% increase in X-ray output for the same average current is obtainableand at 1.5 million volts peak the increase observable is at least 69%.

Thus, this invention contributes a novel solution to the problem ofobtaining increased efliciency and improved quality of the output ofelectron beam apparatus.

While this invention has been described in connection with specificembodiments, by way of example, it will be apparent to those skilled inthe art that it is subject to a wide variety of modifications andalterations without departing from the spirit thereof; therefore, it isintended in the appended claims to cover all modifications andvariations which come within the true spirit and scope of thisinvention.

What I intend to claim and protect by Letters Patent of the UnitedStates is:

1. Apparatus for biasing electron beam equipment, comprising an electrongun having a control electrode means supplying cyclically varyingvoltage to said equipment for accelerating electrons from said gun, saidmeans supplying to said control electrode peaked voltage having properphase, amplitude and periodicity with respect to said acceleratingvoltage so that electrons flow in the beam during a short portion onlyof a cycle of the applied accelerating voltage providing a beam ofelectrons having substantially the same energy.

2. in electron beam equipment, including a source of acceleratingvoltage and an electron gun having a control electrode, apparatus forbiasing said electron beam equipment comprising a peaking transformercoupled to said accelerating voltage source, means connecting saidtransformer to said control electrode, said peaked voltage having properphase and amplitude with respect to said accelerating voltage so thatelectrons flow in said equipment during a short portion only of a cycleof the ap lied accelerating voltage.

3. Apparatus for biasing electron beam equipment, including an electrongun having a control electrode and a source of cyclically varyingvoltage applied to said equipment for accelerating electrons from saidgun, said apparatus comprising a source of peaked voltage connected tosaid control electrode, and a source of bias voltage connected to saidcontrol electrode, said peaked voltage being of proper amplitude withrespect to said bias voltage and of proper phase and periodicity withrespect to said accelerating voltage so that electrons flow in the beamduring a snort portion only of the cycle of the applied acceleratingvoltage.

4. Apparatus for biasing electron beam equipment, including an electrongun having a control and focusing electrode and a source of voltageapplied to said equipment for accelerating electrons from said gun,comprising a peaking transformer, a resistance-capacitance bias networkcoupling said peaking transformer output to said electrode; said.peakedi. voltage havingproper phaseand' toprovide a beam. of: electronshaving substantially. the

same energy..

5.1 Apparatus. for biasing electron. beam equipment, comprising anelectron gun having. a. control electrode, a source of cyclicallyvarying voltage. applied. to' said equipment'for' accelerating.electrons from saidgun, means supplying peaked voltage; rectifying.means connecting saidpeakedi voltage. to said electrode and meanssupplying" bias voltage. to said. electrode; said= peaked voltage being.of proper amplitude; with respectv to said bias voltage and of: properphase; and; periodicity with respect to said accelerating voltage sothatelectrons flow in the beam during a-. short portionqonly' ofa. cycle ofthe applied accelerating voltage.

6; Apparatus-for. biasing electron beam. equipment, including anelectron gun having a control and focusing electrode and a source ofvoltage applied. to said equipment for accelerating electrons from saidgun, comprising, a, peaking transformer, a resistance capacitance biasnetwork and a rectifier, the output of said. transformer, the biasnetwork and said rectifier being connected in circuit. with said.electrode to provide. a. control bias to said electron gun having.substantially no negative voltage peaks and being of. proper phase and.periodicity with respect to said acceleratingvoltage. so. that electronsflow. in said. equipment during a; short portionv only of a cycle of theapplied. accelerating voltage to; provide a beam of electrons havingsubstantially, the same energy.

7. Apparatus. for biasing electron, beam equipment including an electrongun having a. control electrode and a source of cyclically varyingvoltage applied to said equipment for accelerating electrons from saidgun, said apparatus comprising a source of peaked voltage, a source ofsubstantially sine wave voltage having the same periodicity as said.peaked voltage: connected. tov said. peaked voltagev source. to.provide. a resultant bias voltage output for said control electrode sothat current flow in the beam is substantially limited to a. shortportion only of a cycle of the. accelerating voltage.

8. Apparatus for biasing. electron. beam equipment including an electrongun having a control electro-deand a source of cyclically varyingvoltage appliedto said equipment for accelerating electrons; from saidgun,. said apparatus comprising a. source of, peaked. voltage, a sourceof substantially sine wave voltage having the same periodicity as saidpeaked voltage connected in series with 9,. Apparatus. for biasingelectron beam equipment, in cluding, an. electron. gun and a sourcev ofaccelerating voltageconnected. to. said. equipment, to accelerate. elQC:tronsfr'orn said gun, comprising, first and secondmag neticrcores,saidsecond core, normally having a lower reluctance. than said firstcore, a first. winding linking both cores,. means supplying energizingcurrent to said first winding to maintain saidsecond corein, asubstantially saturatedcondition during. a substantialportion of eachcycle of. said energizingcurrent, a second winding linking the second.core, only and a third winding linking both cores, said second and thirdwindings being series connected to provide bias. necessary for limitingbeam current flow. insaidequipmentto a short portion only of the. cycleof the? accelerating, voltage;

10. Apparatus for biasing electron beamequipment, including an,electron. gun, having a control and. focusing electrode and a source ofaccelerating voltage connected to said equipment to accelerate electronsfrom said, gun, comprising first and second magnetic cores, said secondcore normally having a lower reluctance than said first core, a firstwinding linking both cores, means supplying energizing current to. saidfirst winding to maintain said second'core-in a substantially saturatedcondition during a substantial portion of each cycle of said energizingcurrent, asecond winding linkingthe second core only and a third windinglinking both cores, said second and third windings being seriesconnected in phase opposition, and means connecting the combined outputof said second andzthird windings to said electrode to provide the biasnecessary for. limiting current flow in said equipment to ashortihighwoltage portiononly of a cycle. of, the acceleratingvoltageapplied to the equipment to provide a beam ofelectronshavingsubstantially the same energy.

11. A bias source comprising firstv and second magneticcores,..said'secondcore normally having al'ower reluctance thansaidfirst core,.a firstwinding linkingboth cores, said second core,being maintained in a, substantially saturated condition during a,substantial portion of each cycle of said energizing current when saidenergizing current is. applied to said first winding, 2. second windinglinking the second core only and a third winding linking both cores,said second and third windings being series connected to provide shortpositive voltage, peaks during a short portion only of alternate halfcycles of a source of energizing voltage applied to said first winding.

ReferencesCitedin the file of this patent UNITED STATES PATENTS2,133,138 Hamacher Oct. 11, 1938 2,173,221 Ballard Sept. 19, 19392,514,112 Wright et a1. July 4, 1950 2,730,652 Guttonet al Jan. 10, 1956

